A rod mill is a well-known apparatus for making concrete sand, which has a plurality of metal rods rotatably mounted in the milling chamber of a cylindrical drum. In action, as the cylindrical drum rotates, the metal rods crush down material sand to grains of sand by application of impact force. The hardness of the metal rods is far greater than that of the material sand and the impact force applied by the metal rods becomes high. The the metal rods crush the sand with little grinding effect and have no abrasive action which gradually grind down the surface of each material sand grain. Hence, the resultant grain sand produced with such a rod mill is low in quality for use as a finely sized aggregate material for making concrete. On the other hand, natural river sand, mountain sand, sea sand, and land sand are more preferable for use as aggregate sand. Among them, the river sand which is very rigid and almost spherical in the grain shape, exhibits the most desirable quality. It was found through experiment that the percentage of round shaped grains of common water rinsed river sand (calculated through dividing the mass of a unit volume by an absolute dryness specific weight) conforming to JIS A5004 was 57% to 59% while the same of crushed sand produced with a known rod mill was about 53%. Also, grains of the crushed sand are not round but angular and flat in the shape and their surface is not smooth and may have cracks. Those disadvantages will affect the properties of liquid concrete including workability and fluidity.
The applicant invented a novel method of making crushed sand (which was filed in the U.S. Patent Office and issued as U.S. Pat. No.4,995,561 and published by the European Patent Office as EP 0384004 A2).
In more detail, the method is illustrated in FIG. 9 in which material sand 4 is continuously fed together with water 5 into an inner milling chamber 2 of a cylindrical drum 1 which is loaded with a pile of medium stones 3. The material sand 4 is milled in of the cylindrical drum 1 rotated by a drive means to provide milled grains of sand 6, which is in turn unloaded from an exit 7 together with used water. Through the milling action together with water and medium stones, material sand will be ground under a condition similar to that in which river sand is produced from rubble stones by the action of nature. More specifically, the material sand is ground such that impurities on its surface are removed, and the shape of each grain becomes round. The resultant sand is as good as natural river sand, having a quality of an aggregate material which is neither angular nor flat but has a smooth surface which has few or no crack.
However, the concrete sand making method invented by the applicant still has a drawback to be overcome.
For grinding the material sand 4 under the optimum conditions, it is essential that the medium stones 3 which act as abrasives to the material sand 4 are round in shape, like pebbles or ballast stones which are rolled to round shapes in river water. To have such naturally existing round stones or river pebbles in bulk requires large amounts of tough, troublesome labor and time. Since natural round pebbles are limited in amount, they are not practical.
Also, the medium stones 3 are worn partially through the milling action to particles of the milled sand 6. If the particles contain unfavorable substances, they will chemically react with alkali components of a cement thus causing expansion and fracture of the resultant concrete solid of the aggregate sand 6. In addition, steel bars when used for reinforcement of concrete will suffer from corrosion.
When the medium stones 3 exhibit an improper absolute dryness specific weight and are low in the roundness, their pressing or grinding force to the material sand 4 remains less than a desired level, and no optimum abrasive action is obtained. Furthermore, the medium stones 3 become low in the rigidity and hardness and readily break up into bits and pieces.
As described above, it is more preferable to use artificially cracked rubbles rather than naturally ground pebbles in view of amounts of labor or time and the limited amount of natural pebbles. The cracked rubbles however are not round but angular and unsuited for carrying out an abrasive action to material sand.
It is not possible to produce fine grains of sand when such artificially cracked rubbles are used directly with no preparatory action of proper rounding. The cracked, angular rubbles must be rounded.
Meanwhile, the medium stones 3 in the inner milling chamber 2 of the cylindrical drum 1 have to stay a certain level, not too much nor too little, in the overall amount for execution of an optimum grinding action to the material sand 4. As being worn off during the grinding action, the medium stones 3 are reduced with time in the size.
Also, the ratio in supply amount between the material sand 4 and the water 5 is an important factor to enhance the quality of the resultant ground sand 6. If the ratio is improper, the fluidity of the material sand 4 during milling will decline thus preventing even mixture of the material sand 4 and the medium stones 3.
If the rotation per unit time of the cylindrical drum 1 is too fast, the medium stones 3 will be lifted upward along the inner wall of the drum 1 thus decreasing the grinding effect. If it is too slow, the movement of the material sand 4 axially of the cylindrical drum 1 will be retarded thus traveling less smoothly towards the exit 7.